The present invention relates to a method of and an apparatus for recording and reading out radiation image information, and more particularly to such a method of and an apparatus for recording the radiation image information of an object on a stimulable phosphor sheet and detecting light emitted from the stimulable phosphor sheet and representative of the recorded image information by applying stimulating light to the stimulable phosphor sheet, thereby to read the image information, and for converting the same into an electric signal.
When a certain phosphor is exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, the phosphor stores a part of the energy of the radiation. When the phosphor exposed to the radiation is exposed to stimulating rays such as visible light, the phosphor emits light in proportion to the stored energy of the radiation. The phosphor exhibiting such a property is referred to as a "stimulable phosphor".
There has been proposed a radiation image recording and readout system employing such a stimulable phosphor. More specifically, the radiation image of an object such as a human body is stored in a sheet of stimulable phosphor (hereinafter referred to as a "stimulable phosphor sheet" or a "phosphor sheet"), and then the stimulable phosphor sheet is scanned with stimulating rays to cause the stimulable phosphor sheet to emit light representative of the radiation image. The emitted light is then photoelectrically detected to produce an image information signal that is electrically processed for generating image information suitable for diagnostic purpose. The aforesaid radiation image recording and readout system is disclosed in U.S. Pat. No. 4,258,264, Japanese Laid-Open Patent Publication Nos. 55-116340, 55-163472, 56-11395, and 56-104645, for example.
The image which is finally produced can be reproduced as a hard copy, or displayed on a display unit such as a CRT, or recorded on a recording medium such as a magnetic tape for storage over a long period of time. At any rate, the stimulable phosphor sheet does not serve as a final image recording medium, but as a temporary image storage medium for eventually transferring images to the other recording medium or display unit. Therefore, the stimulable phosphor sheet can be used repeatedly, and is highly economical and convenient if in repetitive use.
The radiation image recording and readout apparatus may be installed in a mobile station such as an X-ray photographing unit car, and the mobile station may be set to various places for taking X-ray photographs of a group of people for diagnostic purpose. However, it would be inconvenient for the mobile station to carry many stimulable phosphor sheets, and the number of stimulable phosphor sheets that can be carried on the mobile station or car is limited. It is therefore desirable to carry a reusable stimulable phosphor sheet on the mobile station for recording the radiation image of each object and to store the image signals on a mass storage medium such as a magnetic tape, while the stimulable phosphor sheet is cyclically reused by successively recording and erasing the radiation images. In this manner, the radiation images of many objects or people can be taken on the mobile station within a limited space available. By cyclically reusing the stimulable phosphor sheet, radiation images can successively be taken at an increased speed, so that the images of a group of people can quickly and efficiently be recorded for diagnostic purpose.
To reuse the stimulable phosphor sheet, the remaining radiation energy on the stimulable phosphor sheet after the radiation image has been read out by stimulating rays is discharged by exposure to light or heat, and the stimulable phosphor sheet is employed again for recording a radiation image thereon. The erasure of the irradiation energy from the stimulable phosphor sheet is disclosed in U.S. Pat. No. 4,400,619 and Japanese Laid-Open Patent Publication No. 56-12599, for example.
The applicant has proposed a built-in radiation image recording and readout apparatus (as disclosed in Japanese Patent Application No. 58-66730) which has circulatory feed means for feeding a stimulable phosphor sheet along a circulatory path, an image recording unit disposed in the circulatory path for exposing the sheet to a radiation through an object to record a radiation image on the sheet, an image readout unit disposed in the circulatory path and having a stimulating light source for emitting stimulating light to scan the sheet with the radiation image recorded thereon and photoelectric readout means for detecting light emitted from the sheet scanned by the stimulating light to produce an image signal, and an image erase unit disposed in the circulatory path for discharging remaining radiation energy from the sheet after the image has been read by the image readout unit and before a new radiation image is recorded on the sheet by the image recording unit, the stimulable phosphor sheet being cyclically movable through the units for reuse.
The radiation image recording and readout apparatus of the above arrangement is advantageous in that it can successively and efficiently record and read out radiation image information.
However, in the event that the reusable stimulable phosphor sheet suffers from any surface defect, radiation image information produced therefrom by the image readout unit may be inaccurate, and a doctor may make a wrong diagnosis based on the reproduced image if the imaged object is a patient. Therefore, it is preferable to remove such any surface defect from the stimulable phosphor sheet as soon as possible while the sheet is in cyclic use, or to avoid the use of the sheet, or to replace the stimulable phosphor sheet with another normal stimulable phosphor sheet.
One example of such a surface defect is a scratch on the stimulable phosphor sheet, which prevents a radiation image to be properly recorded on the stimulable phosphor sheet. Any trace of a remaining image or accumulated environmental radiation on a stimulable phosphor sheet as a result of nonuse of the sheet for a long period of time is also responsible for the prevention of a radiation image from being properly recorded on the sheet. When a stimulable phosphor sheet is exposed to a large amount of radiation, any remaining radiation image thereon is so strong as to be detectable as noise, which prevents accurate image information from being produced.
Where there is a scratch on a stimulable phosphor sheet, it is normally removed or the stimulable phosphor sheet is replaced with a new one by a supervisor of the radiation image recording and readout apparatus. The radiation image recording and readout apparatus tends to be less frequently attended by its supervisor, and should no supervisor be available immediately, the scratched stimulable phosphor sheet cannot be removed from the circulatory process at a desired time. The radiation image recording and readout apparatus on the mobile station for taking radiation images of a group of people for diagnostic purpose is more likely to be unattended by its supervisor. Consequently, it is the current practice to reuse a stimulable phosphor sheet, regardless of whether it is scratched or not, in the circulatory feed cycle through the image recording, readout, and erase steps in such system.
Any radiation image information obtained from a defective stimulable phosphor sheet is not accurate enough to be used as proper image information, and hence the image recording, readout, and erase steps effected on such a defective stimulable phosphor sheet may entirely be useless.
Even if there is no scratch on a stimulable phosphor sheet, the sheet may undergo the following problem: After the stimulable phosphor sheet is exposed to erasing light to remove a remaining image therefrom, it may be left unused for a long period of time such as when all image information for one day has been recorded. Then, a trace of radiation energy which may have not been discharged from the sheet by exposure to the erasing light will emerge as a remaining image with time. Radiation energy can also be stored in the sheet by cosmic rays and environmental radiations from another X-ray source. If the remaining image and the energy is thus stored in the sheet while the sheet is left unused for a long time, it is produced as noise when the sheet is reused to record radiation image information. As a consequence, no correct radiation image of an object can be obtained from such stimulable phosphor sheet.
The erase unit of the radiation image recording and readout system serves to emit light or heat toward the stimulable phosphor sheet for thereby causing the sheet to discharge remaining radiation energy. In order to allow the stimulable phosphor sheet to be reusable, the sheet is required to be exposed to more and more light or heat as the remaining radiation energy is greater since the amount of discharged remaining radiation energy is proportional to the amount of erasing light or heat to which the sheet is exposed. Therefore, the amount of erasing light or heat produced in the erase unit is selected to be large enough to discharge the normally expected maximum quantity of remaining radiation energy substantially thoroughly (or to the extent which will not obstruct the next cycle of image recording) from the stimulable phosphor sheet. The amount of erasing light or heat may be controlled dependent on the magnitude of the remaining radiation energy on the sheet with a view to reducing the energy and time required to erase the remaining image. Even in such a case, the maximum controlled amount of erasing light or heat is selected to be sufficiently large to discharge the normally expected maximum quantity of remaining radiation energy substantially thoroughly from the stimulable phosphor sheet.
In taking certain special radiation images, a stimulable phosphor sheet may be exposed to a far greater amount of radiation than normal. The remaining radiation energy on the sheet exposed to such a large amount of radiation may often exceed a normal maximum value after the image has been read from the sheet. When such stimulable phosphor sheet is delivered into the erase unit, it is desirable to expose the sheet to erasing light or heat for a longer interval of time to permit the sheet to be reused for image recording.
In the event that the stimulable phosphor sheet is exposed to erasing light or heat for a longer time for the removal of remaining radiation energy, however, the travel of the sheet has to be arrested in the erase unit, and the image recording and readout steps have to be interrupted.
If the longer time were not consumed for erasing the remaining radiation energy from the sheet, then the remaining radiation energy would tend to be left, and a new radiation image would be recorded on the sheet over the remaining radiation energy. Consequently, the new radiation image recorded on the sheet would not be available as accurate image information.